It is known that certain aircraft comprise a stabilizing horizontal tailplane that can be adjusted in inclination. An adjustable horizontal tailplane such as this is, in the art, designated by one or other of the abbreviations PHR (standing for Plan Horizontal Reglable (i.e., adjustable)) or THS (standing for Trimmable Horizontal Stabilizer). Just like a fixed horizontal tailplane, an adjustable horizontal tailplane is provided with elevators forming the trailing edge of said adjustable horizontal tailplane.
An adjustable horizontal tailplane can be deflected in the nose-up or nose-down direction and it is used in certain flight phases. For example, during the takeoff of the aircraft and prior to rotation, it is customary to deflect said adjustable horizontal tailplane through action of the pilot or of an automatic system, by an angle of deflection of predetermined value. The theoretical optimal value of the angle of deflection of the adjustable horizontal tailplane depends on several parameters of the aircraft, such as the longitudinal position of the center of gravity, the total weight on takeoff, the configuration of the leading edge slats and of the trailing edge flaps, the thrust, the speed of rotation, etc.
The actual value of the angle of deflection is significant since it conditions the behavior of the airplane during the rotation phase, which begins when, the aircraft having through its run reached a predetermined value of speed, the so-called rotation speed, the pilot pulls on the stick so as to actuate said elevators in the nose-up direction and which finishes when the attitude of the aircraft is stabilized around a predetermined value, for example equal to 15°. If the actual value of this angle of deflection is too nose-up, it may lead to spontaneous takeoff without intervention from the pilot or a possible tail touch or else, on the contrary, if it is too nose-down, an arduous takeoff penalizing the performance of the aircraft.
As a general rule, on takeoff, the value of the angle of deflection of the adjustable horizontal tailplane corresponds to a nose-up moment, this being the case in particular when the center of gravity of the aircraft occupies a forward longitudinal position toward the nose of the aircraft: specifically, in this case, the aircraft is difficult to turn at the moment of rotation and the adjustable horizontal tailplane must create a high nose-up moment. However, when the center of gravity of the aircraft is in a rear longitudinal position, the aircraft tends to turn very easily and the adjustable horizontal tailplane must create only a small pitching moment, which may be nose-up, or even nose-down.
As recalled hereinabove, the optimal theoretical value of the angle of deflection, on takeoff, of the adjustable horizontal tailplane depends on numerous parameters. Hence, for an accurate adjustment of the inclination of said adjustable horizontal tailplane, it is necessary to take account of the totality, or at least a large number, of these parameters, thereby leading to complicated adjustment devices.